It is well known to produce continuous fibrillated structures or rovings by suddenly releasing the pressure acting on a two-phase liquid mixture which is based on molten polymer and solvent and which is at a high pressure and a high temperature. These continuous, fibrillated structures are, however, very difficult to process directly, mainly because of their extremely high volume and the rate at which they are produced. For this reason, in order to economically process the continuous, fibrillated structures, they are first subjected to a shredding treatment which reduces their dimensions but which has a detrimental effect on their physical properties and which necessitates tying up a large amount of capital and consuming a great deal of energy.
This is why attempts have been made to develop processes which lead directly to the formation of fibrils of short length when the pressure acting on the above-mentioned mixtures is released suddenly.
Thus, in Belgian Pat. No. 787,032, filed Aug. 1, 1972, which corresponds to U.S. patent application Ser. No. 277,033, assigned to the same assignee as the present application, filed Aug. 1, 1972, entitled "Fabrication of Discontinuous Fibrils", which is hereby incorporated by reference, there is described a process for the direct manufacture of short fibrils in which the two-phase liquid mixture is dispersed in an added fluid at the exact instant when the pressure acting on it is released suddenly. According to Belgian Pat. No. 787,033, filed Aug. 1, 1972, which corresponds to U.S. patent application Ser. No. 277,032, assigned to the same assignee as the present application, filed Aug. 1, 1972, entitled "Process for the Manufacture of Discontinuous Fibrils", which is hereby incorporated by reference, a similar result is achieved when the fibrillated structure formed by suddenly releasing the pressure acting on the two-phase liquid mixture is shredded at the very instant when it is formed, by means of a transverse stream of fluid.
Although these processes very considerably improve the technique for the manufacture of discontinuous fibrils, they nevertheless still possess some disadvantages, the main one of which is that they require the use of very large amounts of added fluid in order to produce adequate shredding.
Consequently, there have been continuing investigations for the purpose of developing a process for the direct manufacture of short fibrils, which no longer requires an added fluid to be supplied.
Thus, in Belgian Pat. No. 811,780, filed Mar. 1, 1974, which corresponds to U.S. patent application Ser. No. 450,475, assigned to the same assignee as the present application, filed Mar. 12, 1974, entitled "Process for the Manufacture of Discontinuous Fibrils", which is hereby incorporated by reference, there is described a process for the manufacture of short fibrils which consists of pulverizing mechanically the ejection cone which forms at the outlet of the orifice by means of which the pressure is suddenly released. This technique, however, involves the use of mechanical devices, such as rotary blades revolving at very high speed and at a very short distance from the pressure release orifice. The result of this is that, although this technique enables good results to be achieved, it is very delicate to carry out, requiring special drive means, and can involve stoppages due to mechanical failure.